1. Field of the Invention
This invention is directed to a generator core monitor that allows a generator to be removed from service before serious damage occurs and, more particularly, to a monitor that detects the development of core hot spots by monitoring the voltage developed across the generator rotor.
2. Description of the Prior Art
A modern generator 8 includes a rotor 14 rotating inside a core 12 surrounded by core windings 13 as illustrated conceptually in FIG. 1. The laminated core 12 of large generators is formed using insulated sheets of ferromagnetic material held together under compression. The surface insulation on the ferromagnetic plates prevents formation of large circulating eddy currents that would be created in the core 12 by the rapidly changing magnetic flux created by the rotor 14. When the insulation breaks down and the insulated ferromagnetic sheets become shorted, large circulating eddy currents are induced in the core 12. The resulting I.sup.2 R heating accelerates failure of the insulation which in turn leads to even higher circulating currents. Severe generator core 12 damage rapidly occurs due to core 12 melting. The melting core 12 can also damage the windings 13 producing a very costly generator failure. If arcing between the stator windings and/or core 12 occurs, the rotor 14 can also be badly damaged.
Prior art generator core monitors are ineffective in detecting generator core failures because the detection mechanism is based on detecting the presence of particulate matter in a circulating hydrogen coolant gas that circulates through the core and windings. A modern core includes inorganic insulation sheets that produce very little particulate matter when breakdown or overheating occurs. Particulate matter in modern generators, when produced, is present only for a brief period, is unevenly mixed with the hydrogen gas, is present only in small concentrations and is difficult to reliably detect. The relationship between the detected level of particulate matter and the severity of core damage is generally not well understood making on-line damage assessment virtually impossible.
Another prior art technique uses temperature measured throughout the generator to determine core problems. Because generator temperature changes slowly even when the core has melting hot spots, this technique will not allow rapid problem identification.
Massive core failure is very expensive to repair because the core must be rewound and as a result early detection of failure is necessary.